In cardiac scanning of the X-ray CT apparatus, in order to obtain a diagnostic image, a cardiac phase in which the motion of the heart is the slowest needs to be timely imaged. In recent years, scanning speed has been improved due to an increase in the rotation speed of the X-ray CT apparatus, an increase in the number of detectors arranged in rows, and the like. However, since the cross-sectional position of the coronary artery of the heart moves about 5 mm to 10 mm in 0.1 second, it is difficult to obtain a clear image without an afterimage even in the current situation. Therefore, it is necessary to create a diagnostic image by collecting the data of the heart rhythm in all cardiac phases and determining an optimal cardiac phase with slowest rhythm by data analysis.
As a technique of determining such an optimal cardiac phase, PTL 1 discloses a method of calculating the variation in the entire heart or a specific location using the data of adjacent cardiac phases and determining a cardiac phase with least motion as an optimal cardiac phase.